Superconducting magnets for storing relatively large energies are currently used in many applications. For example, superconducting magnets, storing energies of up to 10 Mjoules, are being constructed for Magnetic Resonance Imaging (MRI) systems which are now being routinely used in large numbers in clinical environments for medical imaging. A part of such an MRI system is a superconducting solenoid for generating a uniform magnetic field.
Superconducting magnets tend to be inherently unstable in that the temperature of a winding region of the magnet can rise relatively rapidly, due to a malfunction of the magnet itself or due to a cause external to the magnet. Such a temperature rise causes a quenching of that winding region, i.e., the superconducting winding goes from its superconducting state of essentially zero resistance to a finite resistive state. When such region gets hot very rapidly the stored energy within the magnet tends to become dissipated rapidly into that finite resistive region and severely damage the magnet, even in some cases causing an actual melting of the superconducting wires of the winding.
Accordingly, it is necessary to provide protection for the winding, as well as for the winding of the persistent mode switch used in conjunction with the magnet, in order to ensure safe dissipation of the stored energy in case of such an instability. Furthermore, the magnetic field of the system may have to be discharged for reasons other than a malfunction of the magnet itself. For instance, it may be desirable to discharge the magnetic field if a ferromagnetic object is drawn into the strong field region. Because superconducting magnets operate at very low temperatures, they must be thermally isolated from room temperature conditions so that, for purposes of protection or discharge, the currents in the windings thereof cannot be controlled or discharged by an external control unit and only very low currents can be introduced into the low temperature environment of the magnet.
Accordingly, protection and discharge of the magnet are often achieved by the use of heaters which are located both on the windings themselves and on the associated persistent mode switch. If an instability occurs at one particular winding or winding region of the magnet, all of the heaters used thereon are triggered into operation so as to quench all the other regions of the magnet, i.e., the stored energy dissipation does not occur only at the particular winding region where an initial quench has occurred but rather is dissipated throughout the entire magnet and damage to any particular winding region is prevented.
In order to detect such an instability, which results in a rapid quenching of the winding region where the instability occurs, it is necessary to rapidly detect the voltage which results across the winding resistance at a very early stage as the region begins to quench, i.e., when such voltage is relatively small. Otherwise the voltage will build up so rapidly that the power dissipation may well occur before the quench can be detected and before preventive operation can be taken to avoid damage to the magnet.
Currently known techniques for providing early detection of a quench which occurs in a part of a magnet have tended to prove unsatisfactory since they have often required relatively bulky devices for such purpose, which devices cannot be easily used with the magnet structures involved and cannot be satisfactorily arranged to respond rapidly enough to offer the most effective protection for the magnet.